Declutching mechanism with an hydraulic actuator, especially for working in traction on the connecting piece of a clutch

ABSTRACT

A declutching mechanism having firstly, a hydraulic actuator comprising at least one fixed member and a movable member defining a first control chamber, and secondly, a clutch release bearing attached to the movable member and manoeuvrable by the latter in a first axial direction, referred to as the declutching direction, in order to transmit a declutching force to a connecting piece. The actuator includes a second control chamber which is independent of the first control chamber, and which is delimited at least partly by the fixed member and the movable member for the manoeuvre of the movable member in the opposite direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is concerned in general terms with a declutchingmechanism having a clutch release bearing, for working in traction onthe connecting piece of a clutch.

2. Description of the Prior Art

Such a mechanism is described in the document FR-A-2 304 826. In thisdocument the connecting piece, constituting a declutching ring, is partof a primary block which includes the engine and its clutch.

The piece is suitably carried on the declutching means of the clutch,typically the fingers of a diaphragm. The clutch release bearing is partof a secondary block which comprises the gearbox with its input shaft.The release bearing is mounted for axial movement along a guide tube,under the action of a declutching fork which is mounted pivotally on acasing hood fixed to the gearbox.

Tractive coupling means, having a radially elastic coupling member, arearranged between the connecting piece and an actuating member which ispart of a clutch release bearing.

Thus, during the assembly operation, it is necessary to bring theprimary and secondary blocks axially towards each other in order toproceed with fitting them together by snap-fitting or clipping. In thisconnection, bosses are arranged on the release bearing so that the forkcan also drive the release bearing in the opposite direction to thenormal declutching direction.

Thus, by uncoupling the fork from the control linkage, it is possible,due to the coupling member, to cause the fork to deflect in the oppositedirection for the purposes of carrying out the blind snap-fitting of theclutch release bearing with the connecting piece.

These arrangements cannot be used when the clutch release bearing isattached to a hydraulic actuator. For example, as described in thedocument FR-A-2 651 846, the declutching mechanism, for working intraction on the connecting piece of a clutch, in particular for a motorvehicle, includes firstly, a hydraulic actuator comprising at least onefixed member and a movable member in piston and cylinder relationship soas to define a first control chamber, and secondly, a clutch releasebearing attached to the movable member and manoeuvrable by the latter ina first axial direction referred to as a declutching direction, fortransmitting a tractive declutching force to the connecting piece.

In that document, in order to permit snap-fitting assembly, an abutmentis provided which prevents the clutch release bearing from moving duringthe fitting operation.

This abutment is part of a rod having a V-shaped groove at its end.

The rod is fixed to a piston, which is displaceable within a cylinderdefined by a protuberance of the fixed part of the actuator. This rod isacted on by a spring. During the fitting operation, the clutch releasebearing is first retracted manually, and the piston is then displacedmanually to cause the abutment to emerge. Afterwards, the springmaintains the release bearing in position.

After the clutch mechanism has been fitted to the connecting piece, apressure is applied from a master cylinder through a duct in order toretract the rod and the abutment.

All of these arrangements are complicated, take a long time to carryout, and are not compatible with blind fitting. They lead to an increasein they size of the fixed member of the actuator.

OBJECT OF THE INVENTION

An object of the present invention is to overcome these drawbacks, andaccordingly to provide a novel and inexpensive arrangement which enableseasy, rapid, blind snap-fitting of the clutch mechanism having ahydraulic actuator, to its associated connecting piece.

SUMMARY OF THE INVENTION

According to the invention, a clutch mechanism of the type describedabove is characterized in that the hydraulic actuator comprises a secondcontrol chamber, which is independent of the first control chamber anddelimited at least partly by the fixed member and the movable member, tomanoeuvre the movable member and the clutch release bearing axially inthe direction opposite to the declutching direction.

Thanks to the invention, snap-fitting assembly is blindly carried out.In this connection, during the snap-fitting operation it is sufficientto pressurize the second control chamber in such a way that the clutchrelease bearing projects axially with respect to the fixed member.

This pressurization is carried out rapidly, in such a way that assemblytimes are reduced. In addition, the fixed and movable members remainsimple, and use is made of the movable member which is displaced in theopposite direction for snap-fitting the volume of the first chamber thenbeing minimized.

After snap-fitting, the pressure is released in the second controlchamber. This second chamber being independent of the first chamber, itis possible to pressurize it with a fluid separate from the fluid withwhich the first control chamber is pressurize.

It is also possible, after having depressurized the first chamber, topressurize the second chamber so as to release the coupling member andfacilitate the removal of the declutching mechanism.

Thus, by virtue of the second chamber, permanent benefit is obtainedfrom a potential abutment which can easily be made operational bypressurizing the second chamber.

According to an important feature of the invention, the second controlchamber is pressurized pneumatically for the purposes of fitting thedeclutching mechanism on its connecting piece.

Thus it is possible to easily and rapidly to pressurize the secondchamber on the production line, for example with the aid of a compressedair gun, so as to achieve blind fitting.

After snap-fitting has taken place, the gun is withdrawn and the secondcontrol chamber readily evacuates itself naturally and quickly.

In a modification, the second control chamber may be pressurizedpneumatically in advance, and closed by means of a stop valve which isopened after snapfitting has been carried out.

It will be appreciated that this second chamber, pressurizedpneumatically for example with compressed air, constitutes a resilientabutment, and that the pressure which prevails in this second chamber isnot too high, so that during the fitting together of the declutchingmechanism with the connecting pieces there is no risk of damaging thecomponents, while the air does not penetrate into the first controlchamber.

Otherwise when the second chamber is pressurized with an incompressiblefluid, it will give rise to evacuation problems.

A resilient abutment is thus created which is retractable rapidly bydepressurization of the second chamber.

In all cases, the first control chamber is not pressurized. In amodification, the first chamber may be filled beforehand, with its fluidbeing evacuated during fitting or removal.

It will be appreciated that it is possible to continue to use standardconnecting pieces, such as those which are for example described in thedocument FR-A-2 653 195 to which patent U.S. Pat. No. 5,113,989corresponds.

In that case, the fingers of the coupling clip can be unhooked so as toopen before fitting, so that the snap-fitting forces are reduced.

After assembly, the coupling clip is reclosed by hooking its fingerstogether.

It is even possible to elongate the usual engagement aperture of theclutch release bearing for cooperation with its associated couplingclip.

Preferably, the feed orifice for the second chamber is arranged in thevicinity of the zone in which the fixed part is fastened to the gearbox,which simplifies the manufacture of the components.

In one embodiment, the movable member (the piston) is mounted within acavity which is delimited by the fixed member and by a guide tube whichserves for fastening the fixed member.

The piston then preferably has one end having a transverse flange, whichis directed away from the axis of the assembly. This flange thusnaturally delimits its two chambers.

In a modification, the second control chamber is defined on an abutmentsleeve, with which the declutching mechanism is provided for engagementby a return spring. In all cases, use is made of existing components todefine an orifice in a simple way.

It will be appreciated that pressurization of the second chambercompresses the return spring, which enables the coupling member to bedisconnected and facilitates the removal of the declutching means.

In addition, the declutching mechanism may be provided with a protectivebellows which is accessible from outside, and which runs no danger ofbeing torn during the snap-fitting operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view in axial cross section, with the primary and secondaryblocks being shown partially, together with the declutching mechanism inaccordance with the invention after it has been assembled to itsassociated connecting piece.

FIG. 2 is an enlarged view of FIG. 1, showing the declutching mechanismin accordance with the invention on a larger scale.

FIG. 3 is a view in cross section taken on the line 3--3 in FIG. 2.

FIG. 4 is a view in cross section taken on the line 4--4 in FIG. 2.

FIGS. 5 and 6 are views in elevation showing the coupling clip in theclosed and open positions respectively.

FIG. 7 is a view in axial cross section of a declutching mechanism inthe clutch-engaged position, and not equipped with the second controlchamber in accordance with the invention.

FIG. 8 is a view in cross section similar to FIG. 7 showing thedeclutching mechanism, in a second embodiment of the invention, in anextended position before it has been snap-fitted to the connectingpiece.

FIGS. 9 and 10 are diagrammatic views showing the position of the clutchrelease bearing, after it has been snap-fitted to its connecting pieceand during its snap-fitting to the said connecting piece, respectively.

DETAILED DESCRIPTION OF THE INVENTION

These Figures relate to the assembly, by snapfitting, of a declutchingmechanism having a clutch release bearing 15 (FIG. 1), 150 (FIG. 8), ona connecting piece 25 of a clutch 40, of the pull-off type, for a motorvehicle having an internal combustion engine.

The connecting piece 25 is part of a primary block 70 (FIG. 1)comprising the internal combustion engine with its casing 71, while thedeclutching mechanism is part of a secondary block 80, which includesthe gearbox 82 with its hood piece 81 which is adapted to be fixed tothe casing 71.

Tractive fastening means are arranged between the connecting piece 25and the declutching mechanism, for the mounting of the declutchingmechanism, by snapfitting, on its connecting piece 25 which is carriedon the declutching means of the clutch 40. In this example thisdeclutching means consists of a diaphragm 41.

It will be recalled that the clutch 40, apart from the diaphragm 41,comprises a cover plate 42, a pressure plate 43 coupled in rotation tothe cover plate 42 while being mounted for axial movement with respectto the latter, in this example by means of tangential tongues 46, afriction disc 44, and a reaction plate 45.

The group of components 41, 42, 43, which are annular in shape,Constitutes a mechanical clutch which is carried, through its coverplate 42 by means of screws not shown, on the reaction plate 45 which iscoupled in rotation to the crankshaft 73 of the internal combustionengine.

The disc 44 has, at its outer periphery, friction liners which arearranged to be gripped axially between the plates 43 and 45, and, at itsinner periphery, a hub which is internally splined so as to be coupledin rotation with the input shaft 32 of the gearbox.

In the position in which the clutch is engaged, the diaphragm 41 bears,through the outer portion of its Belleville ring, on an engagementsurface carried by the cover plate 42, and through the inner portion ofits Belleville ring, on a divided boss of the pressure plate 43 so as tourge the plate 43 towards the reaction plate 45 and to grip the frictionliners of the disc 44 between the said plates.

The torque is thus transmitted from the crankshaft 73 to the input shaft32.

In order to disengage the clutch, it is necessary to work in traction onthe ends of the fingers of the diaphragm, extending radially inwardlyfrom the inner periphery of the Belleville ring, so as to cause thelatter to deflect and to cause the gripping action exerted on the plate43 to cease, whereby to release the liners of the disc 44.

The manoeuvre is carried out with the aid of a clutch release bearingassembly which includes the connecting piece 25, the declutchingmechanism, and tractive coupling means arranged between the connectingpiece 25 and an actuating element 47 (FIG. 2), 147 (FIG. 8), which ispart of the clutch release bearing.

These means are adapted to effect axial coupling between the components25, 47, going from the declutching device 41 to the clutch releasebearing 15, 150.

They include, firstly, a coupling member 26 which is resilientlydeformable in the radial direction, and which is at least partiallyengaged in an expansion aperture that is formed in one of the componentscomprising the connecting piece and actuating element, and whichprojects in the free state radially with respect to the component.Secondly, a drive surface which is formed in a recess 48, 148 formed onthe other one of the components comprising the actuating element andconnecting piece and with which the coupling member cooperates axiallyin engagement in the axial direction concerned.

All of the arrangements described in the documents FR-A-2 304 826 andFR-A-2 651 846 may be envisaged, but here, the coupling member is anelastic clip of the same type as that described in the document FR-A-2653 195 (U.S. Pat. No. 5,113,989), to which reference is made.

The connecting piece 25 is similar to that in FIG. 3 of U.S. Pat. No.5,113,989. It thus includes a profiled collar which is adapted for itsengagement on the ends of the fingers of the diaphragm 41 integrallywith the collar, a sleeve portion which is arranged to pass axiallythrough the diaphragm, via the central aperture in the latter into whichthe fingers of the diaphragm project.

The coupling clip 26, which is open in the free state, extends annularlyinto the interior of the sleeve portion. It is elastically deformable ina radial direction and is adapted for attachment of the clutch releasebearing 15, 150 by simple snap-fitting of the latter, at least in theaxial sense considered from the collar to the clip, the sleeve portionhaving two distinct openings, each for the passage of a finger of theclip 26, these being separated from each other by a bridge. Thesefingers are directed towards the outside of the sleeve portion, beinghooked one on to the other (FIG. 5).

Each of the fingers is formed with a deformation in the form of a loopwhich elastically increases their length.

The clip also has two lugs (FIGS. 5, 6) in the form of ears, each ofwhich is engaged in an aperture in the sleeve portion.

It is thus possible to perform an unclipping operation by unhooking thefingers (FIG. 6).

The connecting piece 25 is carried on the ends of the fingers of thediaphragm, by means of retaining fingers which are carried incantilevered relationship by lugs which pass through the fingers of thediaphragm, via openings which are arranged between the fingers of thediaphragm.

These retaining fingers extend parallel to the collar in order toensure, in cooperation with the collar of the piece 25, the retention ofthe connecting piece 25 on the diaphragm 41.

In this example, the declutching mechanism includes, firstly, ahydraulic actuator 1, 2, 3-100, 200 comprising at least one fixed member1, 100 and a member 2, 200 which is movable in piston and cylinderrelationship with it, so as to define a first control chamber 50, 500.Secondly, the clutch release bearing 15, 150, which is attached to themovable member 2, 200 and manoeuvrable by the latter in a first axialdirection, referred to as the declutching direction, so as to transmit atractive declutching force to the connecting piece 25.

For assembling of this declutching mechanism together with theconnecting piece, it is necessary to bring the engine towards thegearbox so that the shaft 32 penetrates into the internal bore of thehub of the disc 44, and so that the connecting piece 25 snaps intoengagement with the clutch release bearing 15, 150 by virtue of thecoupling member, the engagement and expansion apertures, and the drivesurface.

To this end, in accordance with the invention, the hydraulic actuator 1,2, 3-100, 200 includes a second control chamber 60, 600 which isindependent of the first control chamber 50, 500, and which is delimitedat least partly by the fixed member 1, 100 and the movable member 2,200, for the manoeuvre of the movable member and the clutch releasebearing axially in a direction opposite to the declutching direction.

In accordance with one feature, the second chamber 60, 600 ispressurized pneumatically for the fitting (or assembly) of thedeclutching means on to the connecting piece. After this fitting hastaken place, the second chamber is depressurized.

In FIGS. 2 and 8, the supply orifice 9, 90 of the second chamber 60, 600is arranged at the level of the zone in which the fixed part 1 isfastened to the gearbox 82 which constitutes a fixed member.

In FIGS. 1 to 4, the clutch release bearing 15 includes a ball bearing14, the inner ring of which is rotatable and defines an actuatingelement 47 having an engagement recess 48, one end of which defines thedrive surface for the clip 26.

The fixed outer ring of this bearing 14 is acted on by a resilient ring18, which in this example is a corrugated ring, and which bears on aradial plate 16 and on one of the sides of the outer ring of the bearing14, so as to bias the other side of this ring into contact with a dishedretaining ring 17, which is attached on the radial plate 16 by a rollingprocess and which for this purpose has at its periphery a notched,axially oriented annular flange.

The bearing 14 is mounted with a radial clearance with respect to thedished retaining ring 17 and to the said annular flange, in such a waythat the release bearing is of the automatic centering type in which itsautomatic centering is maintained.

The radial plate 16 is mounted on the front end portion 19, having areduced diameter, of the piston 2, and is located axially between theshoulder defined by the change in diameter of the piston 2 and arestraining circlip 20 which is fitted in a groove of the end portion19.

The piston 2 constitutes the movable member in accordance with theinvention. It consists of a sleeve, the rear end portion (opposite tothe end portion 19) of which has a transverse flange 52 which isdirected away from the axis of the assembly.

The piston 2 slides axially along the guide tube 3.

This flange penetrates into a cavity which is delimited by a guide tube3 and an annular member 1 in the form of a sleeve, which constitute thefixed part in accordance with the invention.

The members 1 and 3 constitute a cylinder for the piston 2.

For this purpose, the member 1 has at its front end (i.e. that which isclosest to the radial plate 16) a transverse flange 53 which is directedtowards the axis of the assembly, and through the center of which thepiston 2 passes sealingly, while the guide tube 1 has at its rear end atransverse flange 36 described below.

The guide tube 3 serves as a support and centering means for the wholeassembly, especially the piston 2, by preventing reaction forces anddestructive loads from reaching the sliding diameter of the sealingrings 4 and 5, which here are preferably double acting. These rings 4and 5 provide sealing for the chamber 50, which is delimited by theconcentric members 1, 2 and their flanges 52, 53.

A return spring 28 is interposed between the rings 4 and 5. engaging forthis purpose on thrust insert cups 29 which are interposed between eachend of the coil spring and the sealing rings 4 and 5 respectively.

The member 3 has at its end which is adjacent to the gearbox 82 atransverse securing flange 36 which is directed away from the axis ofthe assembly. This flange is shouldered for the purpose of centering themember 1, which has fastening ears 35.

Fastening members 30, which in this example are studs, but which in amodification may be rivets or bolts, pass through the gears 35, and alsopass through the flange 36 so as to secure the fixed assembly (thecylinder) 1, 3 to the fixed member of the vehicle, which in this exampleis the casing of the gearbox 82.

In this example, according to one feature, the second control chamber 60is delimited by the member 1 and the tube 3, and it is closed by theflange 36 and the end flange 52 of the piston 2.

This flange 52 delimits in a simple manner two chambers 50, 60 withinthe cavity defined by the members 1 and 3.

Thus, the second chamber 60 is delimited by the inner periphery 6 of themember 1, by the outer periphery 7 of the member 3, and by the flanges52, 36.

A sealing bellows 27 with notched ends extends between the member 1 andthe radial plate 16, so as to prevent any dirt or other pollutant fromcontaminating the piston 2 at the level of the flange 53 of the member1, to the detriment of its sliding movement.

A preloading spring 21 is also provided, for the purpose of ensuringpermanent contact of the connecting piece 25 with the diaphragm 41, andalso to ensure that the clip 26 is at all times gripped between thesleeve portion of the connecting piece 25 and the engagement surfacedefined in the recess 48 of the actuating element 47 of the releasebearing 15.

This spring 21, which is fitted inside the actuating element 47, actsbetween two thrust elements, each of which is part of a support andguide member 22, 24. These members 22 and 24 are of dished form, thedished member 24 being carried by the piston 2 (the front end portion ofthe latter) and having its base gripped between the end of the coilspring 21 and the circlip 20, while the dished member 22 is carried bythe fixed tube 3 (the front end portion of the latter), being grippedbetween the other end of the spring 21 and a restraining circlip 23which is engaged in a groove provided for this purpose in the tube 3.

The actuating element 47 is thus in constant engagement against the clip26.

A respective supply orifice 93, 9 is associated with each chamber 50,60.

More precisely, the first chamber is fed through a connector 10 and atube 11. The connector 10 is screwed into the threaded orifice 93 of thechamber 50 (FIG. 2).

The second chamber 60 is again fed from a tube, 8, which is open into ahole 9 formed inexpensively in the flange 36, which avoids bothmodification of the member 1 and any increase in size.

The tubes 8 and 11 are connected to a common feed connector 12 which isfixed to a cover 13 that closes off a narrow aperture 83 formed in thehood piece 81.

An access door (not shown) is of course provided in a known manner, inorder to give access to the clip 26, for the purposes of opening andclosing the latter.

The hole 9 is L-shaped, with the tube 8 being force-fitted in itsvertical portion, while its axial portion is blind.

An orifice is provided for purging the chamber 50 (FIG. 3), this beingconnected to a purge tube 33 which communicates into the connector 12(FIGS. 3 and 4).

The connector 12 is secured by means of screws 34 to the cover 13, whichis itself secured by means of screws 34 to the hood piece 81.

Accordingly, in order to proceed with the snapfitting assembly, thechamber 60 is put under pneumatic pressure, using for example acompressed air gun to feed the tube 8. The release bearing 15 then movesforward against the load exerted by the springs 28 and 21, until itcomes into hooking engagement on the connecting piece 25.

It will be noted that all this takes place rapidly and blindly, on theproduction line.

In this connection, it is only after the blocks 70 and 80 have beenbrought together and secured together that compressed air is injectedinto the chamber 60, which causes the piston 2, with the release bearing15, to move forward (towards the left in FIG. 2), and so enablesautomatic assembly to take place by snapfitting to the connecting piece25. After the air gun has been withdrawn, the air escapes, and the tube8 is closed by means of a stopper 54 (FIG. 2), having a central passagefor depressurizing the second chamber 60.

For the purpose of subsequently disassembling the declutching mechanismfrom the piece 25, it is merely necessary, using the access door in thehood piece 81, to open the clip 26 by unhooking its fingers (FIG. 6).

Preferably, after the clip 26 has been opened, the first chamber 50 isdepressurized and the second chamber 60 is pressurized, for example bymeans of the above mentioned air gun, so as to cause the release bearing15 to move forward against the return spring 21, and to release the clip26, which facilitates the disassembly process.

In a modification, the chamber 60 may be put under pneumatic pressure inadvance, and it is during the movement of the two blocks 70 and 80together that the clipping operation is carried out.

In that case, it is necessary to provide a stop valve which enables thechamber 60 to be depressurized after assembly.

It will be appreciated that in all cases, in order to proceed with thefitting of the declutching mechanism to its connecting piece 25, or itsremoval from it, the air or other gas which is contained in the chamber60 is evacuated very easily and quickly, and that the chamber 50 is notpressurized, or at least the fluid contained in the latter is easilyevacuated by an operation in the region of the connector 12 to open thefeed to the chamber.

After the fitting operation has been carried out, the chamber 60 beingdepressurized, it is sufficient to supply the chamber 50 under pressurefrom the tube 11, in order to displace the piston 2 and the releasebearing 15 towards the right in FIG. 2, and to apply traction, by virtueof the clip 26 and the actuating element, to the fingers of thediaphragm 41 so as to cause the latter to deflect.

A rolling bearing 31 is of course interposed between the flange 36 andthe shaft 32, for guiding the latter in rotation.

Similarly, a sealing ring 51 is carried by the piston 2 in the vicinityof its axial end portion 52 which is mounted in the cavity delimited bythe members 1, 3.

In this embodiment, the chambers 50, 60 are arranged axially one behindthe other as in FIG. 8, in which those elements which are identical tothose in FIGS. 1 to 4 carry the same reference numerals increased by100. The control chamber 600 is here defined by the fixed member 100,the movable piston 200, and the thrust and guide members 122, 124 forthe preloading spring 121.

In these Figures, the member 122 acts as an abutment form of a sleevehaving at one of its ends a dished member for abutment and guidance ofthe appropriate end of the spring 121.

The other end of this spring 121 bears on the member transverse outerportion located axially on the piston 200 by means of a circlip 155which is fitted in a groove of the piston 200, a middle portion in theform of a dished element for engagement of the spring 121, and an innerportion defining a radial flange. The sleeve 122 passes through themember 124, with sealing being provided between these two components, asealing ring 156, preferably of a double acting kind, being provided forthat purpose between the two members 122, 124, the ring working betweenthe inner portion of the member 124, in which it is located through itsradial flange, and the outer periphery of the sleeve 122.

A member 157 is secured to the member 100, for example by seaming, andthis member has a sinuous shape having at its inner periphery a sleeveportion joined to a transverse plate portion which is pierced locally soas to accommodate lugs, and so as to be secured by hooking action of thesleeve 122, with the said lugs being upset (at 191).

The supply orifice 90 for the chamber 600 is formed in the vicinity ofthe fixed member 100, and more particularly in the fastening ears ofthis component.

As will have been understood, this fixed member 100 replaces thecomponents 1 and 3, and is fixed directly on to the gearbox 82. Itdefines a cylinder which is concentric with the piston 200.

At its free axial end, the member 100 has a radial flange which isdirected towards the axis of the assembly and which serves for guidingthe piston 200, one of the axial ends of which defines a flange forengagement with the outer ring of the rolling bearing 150, with itsother axial end defining a flange directed away from the axis of theassembly so as to close off the first control chamber 500, which isdelimited directly by the components 100, 200 and the said flanges.

Sealing rings are of course provided, these being shown in grey in theseFigures.

By delivery of compressed air through the orifice 90, the chamber 600 ispressurized so that the release bearing 150 is displaced through thedistance L, and projects with respect to the member 100 defining a 10cylinder.

Thus the actuating element 147 (FIG. 10) penetrates into the interior ofthe connecting piece, and when the chamber 600 is pressurized, the clip26 becomes gripped between the actuating element 147 and the connectingpiece 25 (FIG. 9).

In all cases, the second control chamber 60, 600 is arranged in theinterior of the fixed member, and the two chambers 50, 500 - 60, 600 aredelimited by the movable member 2, 200, being offset axially from each20 other.

During the snap-fitting operation, the volume of the first chamber is ata minimum.

Thus in FIG. 8, the first chamber is not filled.

The present invention is of course not limited to the embodimentdescribed. In particular, the supply orifice for the second chamber maybe located in another position.

In FIG. 8, the structures may be reversed, with the piston 200 thensurrounding the cylinder 100. In that case, the purge connector 192 andthe threaded feed orifice 193 for the first control chamber 500, carriedby the cylinder 100, are located at the rear of the cylinder 100.

It is possible to use any type of conduit (flexible tube, rigid tube orotherwise) for connecting the control chambers 50, 500 - 60, 600 to thecommon feed connector 12.

Finally, and preferably, the sealing rings which are provided for thefirst and second control chambers are of the double acting type in orderto prevent the air from penetrating into the first control chamber, andthe pressure prevailing in the second control chamber is so chosen as toovercome the action of the preloading springs.

I claim:
 1. A declutching mechanism in combination with a connectingpiece of a clutch of a motor vehicle defining an assembly having acentral axis, said mechanism comprising;a hydraulic actuator includingat least one fixed member and a moveable member in piston and cylinderrelationship so as to define a first control chamber; and a clutchrelease bearing attached to said moveable member and manoeuverable bysaid moveable member in a first axial direction defining a declutchingdirection, for transmitting a tractive declutching force to saidconnecting piece; wherein said hydraulic actuator includes a secondcontrol chamber, which is independant of said first control chamber anddeliminated at least partly by said fixed member and said moveablemember for manoeuvering said moveable member and said clutch releasebearing axially in a direction opposite to said declutching direction.2. A mechanism according to claim 1, wherein said control chamber ispressurised pneumatically for the purposes of fitting the declutchingmechanism on said connecting piece.
 3. A mechanism according to claim 2,wherein said second control chamber is connected through a conduit to acommon feed connector, which is also arranged to feed said first controlchamber via another conduit.
 4. A mechanism according to claim 3,wherein said common connector is fixed on a hood piece through a cover.5. A mechanism according to claim 2, wherein said second control chamberis pressurized prior to fitting said mechanism on said connecting piece,said second control chamber being closed by a stop valve for thepurposes of fitting said declutching mechanism on said connecting piece.6. A mechanism according to claim 1, in which the fixed member has azone for its fastening to a second fixed member, wherein said secondcontrol chamber has an orifice located even with a fastening zone ofsaid fixed part.
 7. A mechanism according to claim 1, in which saidmovable member is mounted within a cavity which is defined by said fixedmember and a guide tube for securing said fixed member, wherein saidmovable member has within said cavity an end portion having a firsttransverse flange directed away from said central axis of said assembly,said first transverse flange delimiting said second control chamber(60).
 8. A mechanism according to claim 7, wherein said second controlchamber, delimited by said fixed member and said guide tube, is closedby said first transverse flange of said movable member and by a secondtransverse flange which is part of said tube guide and which is directedaway from said axis of said assembly.
 9. A mechanism according to claim8, wherein said second transverse flange has an L-shaped hole forfeeding said second control chamber.
 10. A mechanism according to claim8, wherein said second transverse flange is shouldered for centeringsaid fixed member, which has securing ears through which there passfastening members, which also pass through said second transverse flangeso as to secure said fixed member and said guide tube to a second fixedmember (82).
 11. A mechanism according to claim 1, wherein said secondcontrol chamber is defined in a sleeve with which said declutchingmechanism is provided for abutment with a return spring.
 12. A mechanismaccording to claim 11, wherein said second control chamber is defined bysaid abutment sleeve, said fixed member, a piston which is movable withrespect to said fixed member, and a second abutment member.
 13. Amechanism according to claim 12, wherein said second abutment member hasa sinuous shape, with a transverse outer portion located axially on saidpiston, a dish-shaped middle portion for engagement with the saidspring, and an inner portion for mounting a ring sealing it with saidabutment sleeve.
 14. A mechanism according to claim 13, wherein saidabutment sleeve is secured in hooked relationship to a member fixed tosaid fixed member.
 15. A mechanism according to claim 1, wherein saidfirst and second control chambers are offset axially with respect toeach other, and being seperated by said movable member.